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MYB46 directly regulates the gene expression of secondary wall-associated cellulose synthases in Arabidopsis - Kim - The Plant Journal - Wiley Online Library

MYB46 directly regulates the gene expression of secondary wall-associated cellulose synthases in Arabidopsis - Kim - The Plant Journal - Wiley Online Library | Arabidopsis | Scoop.it

 

Cellulose is the most abundant biopolymer on Earth. In the secondary cell walls of Arabidopsis, three cellulose synthases (CESA4, CESA7, and CESA8) are necessary for cellulose production. Little is known about how the expression of these CESA genes is regulated. We recently identified a cis-regulatory element (‘M46RE’) recognized by MYB46, which is a master switch for secondary wall formation in Arabidopsis. A genome-wide survey of promoter sequences for the M46RE led to a hypothesis that MYB46 may function as a direct regulator of all three secondary wall-associated cellulose synthases, CESA4, CESA7 and CESA8. We tested this hypothesis using several lines of experimental evidence. All of the three CESA genes are highly upregulated in both constitutive and inducible over-expression of MYB46 in planta. Using a steroid receptor-based inducible activation system, we show that MYB46 directly activates the transcription of the three CESA genes. We then used electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) analysis to confirm that MYB46 protein directly binds the promoters of the three CESA genes both in vitro and in vivo. Furthermore, ectopic upregulation of MYB46 resulted in a significant increase of crystalline cellulose content in Arabidopsis. Taken together, we identify MYB46 as a transcription factor that directly regulates all three secondary wall-associated CESA genes. Yeast-one hybrid screening identified additional transcription factors that regulate the CESAs. However, none of the putative regulators appears to be regulated by MYB46 (Ko et al., 2009; Kim et al., 2012), suggesting multifaceted nature of the transcriptional regulation of secondary wall cellulose biosynthesis.

© 2012 The Authors. The Plant Journal © 2012 Blackwell Publishing Ltd

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Phylloquinone (vitamin K1) biosynthesis in plants: two peroxisomal thioesterases of lactobacillales origin hydrolyze 1,4-dihydroxy-2-naphthoyl-coa (Plant Journal)

Phylloquinone (vitamin K1) biosynthesis in plants: two peroxisomal thioesterases of lactobacillales origin hydrolyze 1,4-dihydroxy-2-naphthoyl-coa (Plant Journal) | Arabidopsis | Scoop.it

It is not known how plants cleave the thioester bond of 1,4-dihydroxy-2-naphthoyl-CoA (DHNA-CoA), a necessary step to form the naphthoquinone ring of phylloquinone (vitamin K1). In fact, only recently has the hydrolysis of DHNA-CoA been demonstrated to be enzyme driven in vivo, and the cognate thioesterase characterized in the cyanobacterium Synechocystis. With a few exceptions in certain prokaryotic (Sorangium and Opitutus) and eukaryotic (Cyanidium, Cyanidioschyzon and Paulinella) organisms, orthologs of DHNA-CoA thioesterase are missing outside of the cyanobacterial lineage. In this study, genomic approaches and functional complementation experiments identified two Arabidopsis genes encoding functional DHNA-CoA thioesterases. The deduced plant proteins display low percentages of identity with cyanobacterial DHNA-CoA thioesterases, and do not even share the same catalytic motif. GFP-fusion experiments demonstrated that the Arabidopsis proteins are targeted to peroxisomes, and subcellular fractionations of Arabidopsis leaves confirmed that DHNA-CoA thioesterase activity occurs in this organelle. In vitro assays with various aromatic and aliphatic acyl-CoA thioester substrates showed that the recombinant Arabidopsis enzymes preferentially hydrolyze DHNA-CoA. Cognate T-DNA knock-down lines display reduced DHNA-CoA thioesterase activity and phylloquinone content, establishing in vivo evidence that the Arabidopsis enzymes are involved in phylloquinone biosynthesis. Extraordinarily, structure-based phylogenies coupled to comparative genomics demonstrate that plant DHNA-CoA thioesterases originate from a horizontal gene transfer with a bacterial species of the Lactobacillales order.

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MORC family ATPases required for heterochromatin condensation and gene silencing (Science)

MORC family ATPases required for heterochromatin condensation and gene silencing (Science) | Arabidopsis | Scoop.it

Transposable elements (TEs) and DNA repeats are commonly targeted by DNA and histone methylation to achieve epigenetic gene silencing. We isolated mutations in two Arabidopsis genes, AtMORC1 and AtMORC6, which cause derepression of DNA-methylated genes and TEs but no losses of DNA or histone methylation. AtMORC1 and AtMORC6 are members of the conserved Microrchidia (MORC) adenosine triphosphatase (ATPase) family, which are predicted to catalyze alterations in chromosome superstructure. The atmorc1 and atmorc6 mutants show decondensation of pericentromeric heterochromatin, increased interaction of pericentromeric regions with the rest of the genome, and transcriptional defects that are largely restricted to loci residing in pericentromeric regions. Knockdown of the single MORC homolog in Caenorhabditis elegans also impairs transgene silencing. We propose that the MORC ATPases are conserved regulators of gene silencing in eukaryotes.

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Arabidopsis Fused kinase and the Kinesin-12 subfamily constitute a signalling module required for phragmoplast expansion (Plant Journal)

Arabidopsis Fused kinase and the Kinesin-12 subfamily constitute a signalling module required for phragmoplast expansion (Plant Journal) | Arabidopsis | Scoop.it

The conserved Fused kinase plays vital but divergent roles in many organisms from Hedgehog signalling in drosophila to polarisation and chemotaxis in Dictyostelium. Previously we have shown that Arabidopsis Fused kinase termed TWO-IN-ONE (TIO) is essential for cytokinesis in both sporophytic and gametophytic cell types. Here using in vivo imaging of GFP-tagged microtubules in dividing microspores we show that TIO is required for expansion of the phragmoplast. We identify the phragmoplast-associated kinesins, PAKRP1/Kinesin-12A and PAKRP1L/Kinesin-12B, as TIO interacting proteins and determine TIO-Kinesin-12 interaction domains and their requirement in male gametophytic cytokinesis. Our results support the role of TIO as a functional protein kinase that interacts with Kinesin-12 subfamily members mainly through the C-terminal ARM repeat domain, but with a contribution from the N-terminal kinase domain. The interaction of TIO with kinesin proteins and the functional requirement of their interaction domains support the operation of a Fused kinase signalling module in phragmoplast expansion that depends upon conserved structural features in diverse Fused kinases.

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Arabidopsis thaliana CENTRORADIALIS homologue (ATC) acts systemically to inhibit floral initiation in Arabidopsis (Plant Journal)

Arabidopsis thaliana CENTRORADIALIS homologue (ATC) acts systemically to inhibit floral initiation in Arabidopsis (Plant Journal) | Arabidopsis | Scoop.it

Floral initiation is orchestrated by systemic floral activators and inhibitors. This remote-control system may integrate environmental cues to modulate floral initiation. Recently, FLOWERING LOCUS T (FT) was found to be a florigen. However, the identity of systemic floral inhibitor or anti-florigen remains to be elucidated. Here we show that Arabidopsis thaliana CENTRORADIALIS homologue (ATC), an Arabidopsis FT homolog, may act in a non-cell autonomous manner to inhibit floral initiation. Analysis of the ATC null mutant revealed that ATC is a short-day induced floral inhibitor. Cell type-specific expression showed that companion cells and apex expressing ATC are sufficient to inhibit floral initiation. Histochemical analysis showed the promoter activity of ATC mainly in vasculature but under the detection limit in apex, which suggests that ATC may moves from the vasculature to the apex to influence flowering. Consistent with this notion, Arabidopsis seedling-grafting experiments demonstrated that ATC moved over long distance and that floral inhibition by ATC is graft transmissible. ATC probably antagonizes FT activity, because both ATC and FT interact with FD and affect the same downstream meristem identity genes APETALA1, in an opposite manner. Thus, photoperiodic variations may trigger functionally opposite FT homologs to systemically influence floral initiation.

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A histone acetyltransferase regulates active DNA demethylation in Arabidopsis (Science)

A histone acetyltransferase regulates active DNA demethylation in Arabidopsis (Science) | Arabidopsis | Scoop.it

Active DNA demethylation is an important part of epigenetic regulation in plants and animals. How active DNA demethylation is regulated and its relationship with histone modification patterns are unclear. Here, we report the discovery of IDM1, a regulator of DNA demethylation in Arabidopsis. IDM1 is required for preventing DNA hypermethylation of highly homologous multicopy genes and other repetitive sequences that are normally targeted for active DNA demethylation by Repressor of Silencing 1 and related 5-methylcytosine DNA glycosylases. IDM1 binds methylated DNA at chromatin sites lacking histone H3K4 di- or trimethylation and acetylates H3 to create a chromatin environment permissible for 5-methylcytosine DNA glycosylases to function. Our study reveals how some genes are indicated by multiple epigenetic marks for active DNA demethylation and protection from silencing.

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PETAL LOSS is a boundary gene that inhibits growth between developing sepals in Arabidopsis thaliana (Plant Journal)

PETAL LOSS is a boundary gene that inhibits growth between developing sepals in Arabidopsis thaliana (Plant Journal) | Arabidopsis | Scoop.it

Flower primordia are partitioned by boundaries during their early development. Such boundaries occur between whorls of organs, and also between organs within whorls. PETAL LOSS (PTL) is a trihelix transcription factor gene that is expressed in boundaries between sepal primordia in the outer whorl. Over-expression of PTL results in growth suppression suggesting that PTL normally inhibits growth between newly arising sepals. We have tested this by examining the consequences of loss of PTL function using confocal imaging. The size of the inter-sepal zone in stage 4 buds expands radially by 35–40% in ptl-1 mutants as a consequence of additional cell proliferation. There is no change in the size of PTL-expressing cells. PTL expression does not overlap with the sites of petal initiation identified using the DR5 auxin response reporter. The latter are closer to the centre of the flower. Thus the consequence of loss of PTL function on petal initiation is indirect, perhaps through interference with a mobile petal-initiation signal or movement of the PTL protein. CUP-SHAPED COTYLEDON (CUC) genes are also involved in defining inter-sepal boundaries. However, genetic studies combining ptl with loss of cuc1 function, and gain of CUC function in extra early petals-1 (miR164c) mutants, have revealed that CUC and PTL act differently. CUC suppresses growth of sepal tissues from the boundary region whereas PTL acts to keep the size of the boundary in check.

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ISWI chromatin remodeling factors and their interacting RINGLET proteins act together in controlling the plant vegetative phase in Arabidopsis (Plant Journal)

ISWI chromatin remodeling factors and their interacting RINGLET proteins act together in controlling the plant vegetative phase in Arabidopsis (Plant Journal) | Arabidopsis | Scoop.it

During their life cycle, flowering plants must experience a transition from vegetative to reproductive growth. Here, we report that double mutations in the Arabidopsis thaliana IMITATION SWITCH (AtISWI) genes, CHROMATIN REMODELING11 (CHR11) and CHR17, and the plant-specific DDT-domain containing genes, RINGLET1 (RLT1) and RLT2, resulted in plants with similar developmental defects, including the dramatically accelerated vegetative-to-reproductive transition. We demonstrated that AtISWI physically interacts with RLTs in preventing plants from early activation of the vegetative-to-reproductive transition by regulating several key genes that contribute to flower timing. In particular, AtISWI and RLTs repress FT, SEP1, SEP3, FUL, and SOC1, but promote FLC in the leaf. Furthermore, AtISWI and RLTs may directly repress FT and SEP3 through associating with the FT and SEP3 loci. Our study reveals that AtISWI and RLTs represent a previously unrecognized genetic pathway that is required for the maintenance of the plant vegetative phase.

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The Arabidopsis thaliana DSB FORMATION (AtDFO) gene is required for meiotic double strand break formation (Plant Journal)

The Arabidopsis thaliana DSB FORMATION (AtDFO) gene is required for meiotic double strand break formation (Plant Journal) | Arabidopsis | Scoop.it

DNA Double Strand Break (DSB) formation is the initial event for meiotic recombination catalyzed by the conserved Spo11 protein. In Arabidopsis, several proteins have been reported to be involved in DSB formation. Here, we report a ArabidopsisDSB Forming (DFO) gene in Arabidopsis that is involved in DSB formation. The dfo mutant exhibits reduced fertility, producing polyads with an abnormal number of microspores, unlike the tetrads in the wild type. The dfo meiocytes were defective in homologous chromosome synapsis and segregation. Genetic analysis revealed that the homologous recombination of Atdfo-1 is severely affected in meiotic prophase I. DFO encodes a protein without any known conserved domain. There was no homologue identified outside the plant kingdom, indicating that AtDFO is a plant-specific protein. AtMRE11 has been reported to be responsible for processing SPO11-generated DSBs. The Atmre11 mutant displays chromosome fragmentation during meiosis. However, the Atdfo Atmre11 double mutant had no such chromosome fragmentation, indicating that AtDFO is required for DSB formation.

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Identification of an abscisic acid transporter by functional screening using the receptor complex as a sensor (PNAS)

Identification of an abscisic acid transporter by functional screening using the receptor complex as a sensor (PNAS) | Arabidopsis | Scoop.it

Movement of the plant hormone abscisic acid (ABA) within plants has been documented; however, the molecular mechanisms that regulate ABA transport are not fully understood. By using a modified yeast two-hybrid system, we screened Arabidopsis cDNAs capable of inducing interactions between the ABA receptor PYR/PYL/RCAR and PP2C protein phosphatase under low ABA concentrations. By using this approach, we identified four members of the NRT1/PTR family as candidates for ABA importers. Transport assays in yeast and insect cells demonstrated that at least one of the candidates ABA-IMPORTING TRANSPORTER (AIT) 1, which had been characterized as the low-affinity nitrate transporter NRT1.2, mediates cellular ABA uptake. Compared with WT, the ait1/nrt1.2 mutants were less sensitive to exogenously applied ABA during seed germination and/or postgermination growth, whereas overexpression of AIT1/NRT1.2 resulted in ABA hypersensitivity in the same conditions. Interestingly, the inflorescence stems of ait1/nrt1.2 had a lower surface temperature than those of the WT because of excess water loss from open stomata. We detected promoter activities of AIT1/NRT1.2 around vascular tissues in inflorescence stems, leaves, and roots. These data suggest that the function of AIT1/NRT1.2 as an ABA importer at the site of ABA biosynthesis is important for the regulation of stomatal aperture in inflorescence stems.

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ABP1 and ROP6 GTPase signaling regulate clathrin-mediated endocytosis in Arabidopsis roots (Current Biology)

ABP1 and ROP6 GTPase signaling regulate clathrin-mediated endocytosis in Arabidopsis roots (Current Biology) | Arabidopsis | Scoop.it

The dynamic spatial and temporal distribution of the crucial plant signaling molecule auxin is achieved by feedback coordination of auxin signaling and intercellular auxin transport pathways. Developmental roles of auxin have been attributed predominantly to its effect on transcription; however, an alternative pathway involving AUXIN BINDING PROTEIN1 (ABP1) has been proposed to regulate clathrin-mediated endocytosis in roots and Rho-like GTPase (ROP)-dependent pavement cell interdigitation in leaves. In this study, we show that ROP6 and its downstream effector RIC1 regulate clathrin association with the plasma membrane for clathrin-mediated endocytosis, as well as for its feedback regulation by auxin. Genetic analysis revealed that ROP6/RIC1 acts downstream of ABP1 to regulate endocytosis. This signaling circuit is also involved in the feedback regulation of PIN-FORMED 1 (PIN1) and PIN2 auxin transporters activity (via its constitutive endocytosis) and corresponding auxin transport-mediated processes, including root gravitropism and leave vascular tissue patterning. Our findings suggest that the signaling module auxin–ABP1–ROP6/RIC1–clathrin–PIN1/PIN2 is a shared component of the feedback regulation of auxin transport during both root and aerial development.

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CK2-defective Arabidopsis plants exhibit enhanced double-strand break repair rates and reduced survival after exposure to ionizing radiation (Plant Journal)

CK2-defective Arabidopsis plants exhibit enhanced double-strand break repair rates and reduced survival after exposure to ionizing radiation (Plant Journal) | Arabidopsis | Scoop.it

The multifunctional protein kinase CK2 is involved in several aspects of the DNA damage response (DDR) in mammals. To gain insight into the role of CK2 in plant genome maintenance, we studied the response to genotoxic agents of an Arabidopsis CK2 dominant-negative mutant (CK2mut plants). CK2mut plants were hypersensitive to a wide range of genotoxins that produce a variety of DNA lesions. However, they were able to activate the DDR after exposure to γ irradiation, as shown by accumulation of phosphorylated histone H2AX and up-regulation of sets of radio-modulated genes. Moreover, functional assays showed that mutant plants quickly repair the DNA damage produced by genotoxins, and that they exhibit preferential use of non-conservative mechanisms, which may explain plant lethality. The chromatin of CK2mut plants was more sensitive to digestion with micrococcal nuclease, suggesting compaction changes that agreed with the transcriptional changes detected for a number of genes involved in chromatin structure. Furthermore, CK2mut plants were prone to transcriptional gene silencing release upon genotoxic stress. Our results suggest that CK2 is required in the maintenance and control of genomic stability and chromatin structure in plants, and that this process affects several functions, including the DNA damage response and DNA repair.

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Chromatin regulation of flowering (TiPSc)

Chromatin regulation of flowering (TiPSc) | Arabidopsis | Scoop.it

The transition to flowering is a major developmental switch in the life cycle of plants. In Arabidopsis (Arabidopsis thaliana), chromatin mechanisms play critical roles in flowering-time regulation through the expression control of key flowering-regulatory genes. Various conserved chromatin modifiers, plant-specific factors, and long noncoding RNAs are involved in chromatin regulation of FLOWERING LOCUS C (FLC, a potent floral repressor). The well-studied FLC regulation has provided a paradigm for chromatin-based control of other developmental genes. In addition, chromatin modification plays an important role in the regulation of FLOWERING LOCUS T (FT, encoding florigen), which is widely conserved in angiosperm species. The chromatin mechanisms underlying FT regulation in Arabidopsis are likely involved in the regulation of FT relatives and, therefore, flowering-time control in other plants.

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Casparian strip diffusion barrier in Arabidopsis is made of a lignin polymer without suberin (PNAS)

Casparian strip diffusion barrier in Arabidopsis is made of a lignin polymer without suberin (PNAS) | Arabidopsis | Scoop.it

Casparian strips are ring-like cell-wall modifications in the root endodermis of vascular plants. Their presence generates a paracellular barrier, analogous to animal tight junctions, that is thought to be crucial for selective nutrient uptake, exclusion of pathogens, and many other processes. Despite their importance, the chemical nature of Casparian strips has remained a matter of debate, confounding further molecular analysis. Suberin, lignin, lignin-like polymers, or both, have been claimed to make up Casparian strips. Here we show that, in Arabidopsis, suberin is produced much too late to take part in Casparian strip formation. In addition, we have generated plants devoid of any detectable suberin, which still establish functional Casparian strips. In contrast, manipulating lignin biosynthesis abrogates Casparian strip formation. Finally, monolignol feeding and lignin-specific chemical analysis indicates the presence of archetypal lignin in Casparian strips. Our findings establish the chemical nature of the primary root-diffusion barrier in Arabidopsis and enable a mechanistic dissection of the formation of Casparian strips, which are an independent way of generating tight junctions in eukaryotes.

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A histone acetyltransferase regulates active DNA demethylation in Arabidopsis (Science)

A histone acetyltransferase regulates active DNA demethylation in Arabidopsis (Science) | Arabidopsis | Scoop.it

Active DNA demethylation is an important part of epigenetic regulation in plants and animals. How active DNA demethylation is regulated and its relationship with histone modification patterns are unclear. Here, we report the discovery of IDM1, a regulator of DNA demethylation in Arabidopsis. IDM1 is required for preventing DNA hypermethylation of highly homologous multicopy genes and other repetitive sequences that are normally targeted for active DNA demethylation by Repressor of Silencing 1 and related 5-methylcytosine DNA glycosylases. IDM1 binds methylated DNA at chromatin sites lacking histone H3K4 di- or trimethylation and acetylates H3 to create a chromatin environment permissible for 5-methylcytosine DNA glycosylases to function. Our study reveals how some genes are indicated by multiple epigenetic marks for active DNA demethylation and protection from silencing.

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Homologous RXLR effectors from Hyaloperonospora arabidopsidis and Phytophthora sojae suppress immunity in distantly related plants (Plant Journal)

Homologous RXLR effectors from Hyaloperonospora arabidopsidis and Phytophthora sojae suppress immunity in distantly related plants (Plant Journal) | Arabidopsis | Scoop.it

Diverse pathogens secrete effector proteins into plant cells to manipulate host cellular processes. Oomycete pathogens contain large complements of predicted effector genes defined by an RXLR host cell entry motif. The genome of Hyaloperonospora arabidopsidis (Hpa, downy mildew of Arabidopsis) contains at least 134 candidate RXLR effector genes. Only a small subset of these genes is conserved in related oomycetes from the Phytophthora genus. Here, we describe a comparative functional characterization of the Hpa RXLR effector gene HaRxL96 and a homologous gene, PsAvh163, from the soybean pathogen Phytophthora sojae. HaRxL96 and PsAvh163 are induced during early stages of infection and carry a functional RXLR motif that is sufficient for protein uptake into plant cells. Both effectors can suppress immune responses in soybean. HaRxL96 suppresses immunity in Nicotiana benthamiana, while PsAvh163 induces an HR-like cell death response in Nicotiana that is dependent on RAR1 and Hsp90.1. Transgenic Arabidopsis plants expressing HaRxL96 or PsAvh163 exhibit elevated susceptibility to virulent and avirulent Hpa as well as decreased callose deposition in response to non-pathogenic P. syringae. Both effectors interfere with defense marker gene induction, but do not affect salicylic acid biosynthesis. Together, these experiments demonstrate that evolutionarily conserved effectors from different oomycete species can suppress immunity in plant species that are divergent from the source pathogen’s host.

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An efficient multi-locus mixed-model approach for genome-wide association studies in structured populations (Nat Genet)

An efficient multi-locus mixed-model approach for genome-wide association studies in structured populations (Nat Genet) | Arabidopsis | Scoop.it

Population structure causes genome-wide linkage disequilibrium between unlinked loci, leading to statistical confounding in genome-wide association studies. Mixed models have been shown to handle the confounding effects of a diffuse background of large numbers of loci of small effect well, but they do not always account for loci of larger effect. Here we propose a multi-locus mixed model as a general method for mapping complex traits in structured populations. Simulations suggest that our method outperforms existing methods in terms of power as well as false discovery rate. We apply our method to human and Arabidopsis thaliana data, identifying new associations and evidence for allelic heterogeneity. We also show how a priori knowledge from an A. thaliana linkage mapping study can be integrated into our method using a Bayesian approach. Our implementation is computationally efficient, making the analysis of large data sets (n > 10,000) practicable.

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Nitric oxide promotes MPK6-mediated caspase-3-like activation in cadmium-induced Arabidopsis thaliana programmed cell death (Plant, Cell & Environment)

Nitric oxide promotes MPK6-mediated caspase-3-like activation in cadmium-induced Arabidopsis thaliana programmed cell death (Plant, Cell & Environment) | Arabidopsis | Scoop.it

Nitric oxide (NO), a vital cell-signalling molecule, has been reported to regulate toxic metal responses in plants. This work investigated the effects of NO and the relationship between NO and mitogen-activated protein kinase (MAPK) in Arabidopsis (Arabidopsis thaliana) programmed cell death (PCD) induced by cadmium (Cd2+) exposure. With fluorescence resonance energy transfer (FRET) analysis, caspase-3-like protease activation was detected after Cd2+ treatment. This was further confirmed with a caspase-3 substrate assay. Cd2+-induced caspase-3-like activity was inhibited in the presence of the NO-specific scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), suggesting that NO mediated caspase-3-like protease activation under Cd2+ stress conditions. Pretreatment with cPTIO effectively inhibited Cd2+-induced MAPK activation, indicating that NO also affected the MAPK pathway. Interestingly, Cd2+-induced caspase-3-like activity was significantly suppressed in the mpk6 mutant, suggesting that MPK6 was required for caspase-3-like protease activation. To our knowledge, this is the first demonstration that NO promotes Cd2+-induced Arabidopsis PCD by promoting MPK6-mediated caspase-3-like activation.

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Histone H1 affects gene imprinting and DNA methylation in Arabidopsis (Plant Journal)

Histone H1 affects gene imprinting and DNA methylation in Arabidopsis (Plant Journal) | Arabidopsis | Scoop.it

Imprinting, i.e. parent-of-origin expression of alleles, plays an important role in regulating development in mammals and plants. DNA methylation catalyzed by DNA methyltransferases plays a pivotal role in regulating imprinting by silencing parental alleles. DEMETER (DME), a DNA glycosylase functioning in the base-excision DNA repair pathway, can excise 5-methylcytosine from DNA and regulate genomic imprinting in Arabidopsis. DME demethylates the maternal MEDEA (MEA) promoter in endosperm, resulting in expression of the maternal MEA allele. However, it is not known whether DME interacts with other proteins in regulating gene imprinting. Here we report the identification of histone H1.2 as a DME-interacting protein in a yeast two-hybrid screen, and confirmation of their interaction by the in vitro pull-down assay. Genetic analysis of the loss-of-function histone h1 mutant showed that the maternal histone H1 allele is required for DME regulation of MEA, FWA and FIS2 imprinting in Arabidopsis endosperm but the paternal allele is dispensable. Furthermore, we show that mutations in histone H1 result in an increase of DNA methylation in the maternal MEA and FWA promoter in endosperm. Our results suggest that histone H1 is involved in DME-mediated DNA methylation and gene regulation at imprinted loci.

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Role of actin cytoskeleton in brassinosteroid signaling and in its integration with the auxin response in plants (Developmental Cell)

Role of actin cytoskeleton in brassinosteroid signaling and in its integration with the auxin response in plants (Developmental Cell) | Arabidopsis | Scoop.it

In plants, developmental programs and tropisms are modulated by the phytohormone auxin. Auxin reconfigures the actin cytoskeleton, which controls polar localization of auxin transporters such as PIN2 and thus determines cell-type-specific responses. In conjunction with a second growth-promoting phytohormone, brassinosteroid (BR), auxin synergistically enhances growth and gene transcription. We show that BR alters actin configuration and PIN2 localization in a manner similar to that of auxin. We describe a BR constitutive-response mutant that bears an allele of the ACTIN2 gene and shows altered actin configuration, PIN2 delocalization, and a broad array of phenotypes that recapitulate BR-treated plants. Moreover, we show that actin filament reconfiguration is sufficient to activate BR signaling, which leads to an enhanced auxin response. Our results demonstrate that the actin cytoskeleton functions as an integration node for the BR signaling pathway and auxin responsiveness.

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Aberrant temporal growth pattern and morphology of root and shoot caused by a defective circadian clock in Arabidopsis thaliana (Plant Journal)

Aberrant temporal growth pattern and morphology of root and shoot caused by a defective circadian clock in Arabidopsis thaliana (Plant Journal) | Arabidopsis | Scoop.it

Circadian clocks synchronized with the environment allow plants to anticipate recurring daily changes and give a fitness advantage. Here, we mapped the dynamic growth phenotype of leaves and roots in two lines of Arabidopsis thaliana with a disrupted circadian clock: the CCA1-overexpressing line (CCA1ox) and the prr9-prr7-prr5 (prr975) mutant. We demonstrate, on a diel (24 h) time scale, the leaf growth defects due to a disrupted circadian clock. Both lines showed enhanced leaf growth compared with the wild type during the diurnal period, suggesting increased partitioning of photosynthates for leaf growth. Nocturnal leaf growth was reduced and growth inhibition occurred by dawn, which can be explained by ineffective starch degradation in the leaves of the mutants. However, this growth inhibition was not caused by starch exhaustion. Overall, these results are consistent with the notion that the defective clock affects carbon and energy allocation, thereby reducing growth capacity during the night. Furthermore, rosette morphology and size as well as root architecture were strikingly altered by the defective clock control. Separate analysis of the primary root and lateral roots revealed strong suppression of lateral root formation in both CCA1ox and prr975, which was accompanied by peculiar changes in lateral root growth direction in light-dark cycles and increased lateral extension of the root system. We conclude that growth of the whole plant is severely affected by improper clock regulation in A. thaliana, resulting in not only altered timing and capacity of growth but also aberrant development of shoot and root architecture.

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Reconstitution of abscisic acid activation of SLAC1 anion channel by CPK6 and OST1 kinases and branched ABI1 PP2C phosphatase action (PNAS)

Reconstitution of abscisic acid activation of SLAC1 anion channel by CPK6 and OST1 kinases and branched ABI1 PP2C phosphatase action (PNAS) | Arabidopsis | Scoop.it

The plant hormone abscisic acid (ABA) is produced in response to abiotic stresses and mediates stomatal closure in response to drought via recently identified ABA receptors (pyrabactin resistance/regulatory component of ABA receptor; PYR/RCAR). SLAC1 encodes a central guard cell S-type anion channel that mediates ABA-induced stomatal closure. Coexpression of the calcium-dependent protein kinase 21 (CPK21), CPK23, or the Open Stomata 1 kinase (OST1) activates SLAC1 anion currents. However, reconstitution of ABA activation of any plant ion channel has not yet been attained. Whether the known core ABA signaling components are sufficient for ABA activation of SLAC1 anion channels or whether additional components are required remains unknown. The Ca2+-dependent protein kinase CPK6 is known to function in vivo in ABA-induced stomatal closure. Here we show that CPK6 robustly activates SLAC1-mediated currents and phosphorylates the SLAC1 N terminus. A phosphorylation site (S59) in SLAC1, crucial for CPK6 activation, was identified. The group A PP2Cs ABI1, ABI2, and PP2CA down-regulated CPK6-mediated SLAC1 activity in oocytes. Unexpectedly, ABI1 directly dephosphorylated the N terminus of SLAC1, indicating an alternate branched early ABA signaling core in which ABI1 targets SLAC1 directly (down-regulation). Furthermore, here we have successfully reconstituted ABA-induced activation of SLAC1 channels in oocytes using the ABA receptor pyrabactin resistant 1 (PYR1) and PP2C phosphatases with two alternate signaling cores including either CPK6 or OST1. Point mutations in ABI1 disrupting PYR1–ABI1 interaction abolished ABA signal transduction. Moreover, by addition of CPK6, a functional ABA signal transduction core from ABA receptors to ion channel activation was reconstituted without a SnRK2 kinase.

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Heterosis manifestation during early Arabidopsis seedling development is characterized by intermediate gene expression and enhanced metabolic activity in the hybrids (Plant Journal)

Heterosis manifestation during early Arabidopsis seedling development is characterized by intermediate gene expression and enhanced metabolic activity in the hybrids (Plant Journal) | Arabidopsis | Scoop.it

Heterosis-associated cellular and molecular processes were analyzed in seeds and seedlings of Arabidopsis thaliana accessions Col-0 and C24 and their heterotic hybrids. Microscopic examination revealed no advantages in terms of hybrid mature embryo organ sizes or cell numbers. Increased cotyledon sizes were detectable 4 days after sowing. Growth heterosis results from elevated cell sizes and numbers, and is well established at 10 days after sowing. The relative growth rates of hybrid seedlings were most enhanced between 3 and 4 days after sowing. Global metabolite profiling and targeted fatty acid analysis revealed maternal inheritance patterns for a large proportion of metabolites in the very early stages. During developmental progression, the distribution shifts to dominant, intermediate and heterotic patterns, with most changes occurring between 4 and 6 days after sowing. The highest incidence of heterotic patterns coincides with establishment of size differences at 4 days after sowing. In contrast, overall transcript patterns at 4, 6 and 10 days after sowing are characterized by intermediate to dominant patterns, with parental transcript levels showing the largest differences. Overall, the results suggest that, during early developmental stages, intermediate gene expression and higher metabolic activity in the hybrids compared to the parents lead to better resource efficiency, and therefore enhanced performance in the hybrids.

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A ROP GTPase-dependent auxin signaling pathway regulates the subcellular distribution of PIN2 in Arabidopsis roots (Current Biology)

A ROP GTPase-dependent auxin signaling pathway regulates the subcellular distribution of PIN2 in Arabidopsis roots (Current Biology) | Arabidopsis | Scoop.it

PIN-FORMED (PIN) protein-mediated auxin polar transport is critically important for development, pattern formation, and morphogenesis in plants. Auxin has been implicated in the regulation of polar auxin transport by inhibiting PIN endocytosis, but how auxin regulates this process is poorly understood. Our genetic screen identified the Arabidopsis SPIKE1 (SPK1) gene whose loss-of-function mutations increased lateral root density and retarded gravitropic responses, as do pin2 knockout mutations. SPK1 belongs to the conserved DHR2-Dock family of Rho guanine nucleotide exchange factors [4,5,6]. The spk1 mutations induced PIN2 internalization that was not suppressed by auxin, as did the loss-of-function mutations for Rho-like GTPase from Plants 6 (ROP6)-GTPase or its effector RIC1. Furthermore, SPK1 was required for auxin induction of ROP6 activation. Our results have established a Rho GTPase-based auxin signaling pathway that maintains PIN2 polar distribution to the plasma membrane via inhibition of its internalization in Arabidopsis roots. Our findings provide new insights into signaling mechanisms that underlie the regulation of the dynamic trafficking of PINs required for long-distance auxin transport and that link auxin signaling to PIN-mediated pattern formation and morphogenesis.

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Casparian strip diffusion barrier in Arabidopsis is made of a lignin polymer without suberin (PNAS)

Casparian strip diffusion barrier in Arabidopsis is made of a lignin polymer without suberin (PNAS) | Arabidopsis | Scoop.it

Casparian strips are ring-like cell-wall modifications in the root endodermis of vascular plants. Their presence generates a paracellular barrier, analogous to animal tight junctions, that is thought to be crucial for selective nutrient uptake, exclusion of pathogens, and many other processes. Despite their importance, the chemical nature of Casparian strips has remained a matter of debate, confounding further molecular analysis. Suberin, lignin, lignin-like polymers, or both, have been claimed to make up Casparian strips. Here we show that, in Arabidopsis, suberin is produced much too late to take part in Casparian strip formation. In addition, we have generated plants devoid of any detectable suberin, which still establish functional Casparian strips. In contrast, manipulating lignin biosynthesis abrogates Casparian strip formation. Finally, monolignol feeding and lignin-specific chemical analysis indicates the presence of archetypal lignin in Casparian strips. Our findings establish the chemical nature of the primary root-diffusion barrier in Arabidopsis and enable a mechanistic dissection of the formation of Casparian strips, which are an independent way of generating tight junctions in eukaryotes.

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